1. Field of the Invention
The invention concerns a method, a magnetic resonance apparatus and a computer program for an acquisition of measurement data of an examination region of an examination subject, in particular a patient, during a continuous travel of the examination region through a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance (MR) is a known modality with which images of the inside of an examination subject can be generated. Expressed simply, the examination subject is positioned in a comparably strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla to 7 Tesla or more) in a magnetic resonance apparatus so that its nuclear spins orient along the basic magnetic field. Radio-frequency excitation pulses are radiated into the examination subject, that cause the nuclear spins to behave so as to emit magnetic resonance signals that are measured and MR images are reconstructed based thereon. For spatial coding of the measurement data, rapidly-switched (activated) magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix populated with such values, for example by means of a multidimensional Fourier transformation. The examination subject can be living (for example an animal or a patient) or inanimate (for example a sample or a phantom).
Magnetic resonance apparatuses with a support device (for example a patient bed) that can be automatically driven into and out of a patient receptacle (by means of a drive device) of the magnetic resonance apparatus that is permeated by a magnetic field of the magnetic resonance apparatus are known for the acquisition of magnetic resonance images. Since the patient receptacle frequently has a quite small diameter, the patient is placed on the patient bed outside of the patient receptacle, after which the patient bed can be automatically driven into the patient receptacle by means of the drive device.
The patient or another examination subject is briefly, continuously driven through the magnetic resonance apparatus by means of the support device during the acquisition of the measurement data from an examination region of the patient, or the examination subject. The measured “field of view” (FOV) can be expanded in the direction of the travel direction of the support device by controlling the movement of the support device, so examination regions that are larger in the direction of the travel direction of the support device than the measurement volume of the magnetic resonance apparatus can be examined. For example, whole-body acquisitions of patients can be generated in one measurement pass. Conversely, the measurement volume in which optimally ideal measurement conditions are generated can be limited in the direction of the travel direction of the support without limiting the total achievable FOV.
Applied techniques for such an acquisition of measurement data can be roughly subdivided into two-dimensional (2D) axial measurements with the travel direction of the support device perpendicular to the readout direction of the measurement data, and three-dimensional (3D) techniques in which the readout direction of the measurement data is oriented parallel to the travel direction of the support device. An overview of such techniques is provided in, for example, the article by Börnert and Aldefeld, “Principles of Whole-Body Continuously-Moving-Table MRI”, Journal of Magnetic Resonance Imaging 28: 1-12 (2008).
In the following the cited 2D axial measurements are discussed in detail. Given these the examination region is divided into and measured in slices that are situated perpendicular to the movement direction of the support device. In the simplest case, these slices are measured sequentially in the center of the magnetic resonance apparatus. In specific sequences (for example multi-shot sequences with a TR (TR: “repetition time”) interval of a few tenths of a millisecond or longer) the sequential measurement requires a slow speed in the movement of the support device, which leads to long total measurement times and is therefore ineffective. One possibility to accelerate the measurement is to combine adjacent slices into slice stacks and to measure the slices of a slice stack in an interleaved manner (as in static measurement, i.e. without movement of the support device during the measurement). The slice stacks themselves are measured in succession. During the measurement of a slice, the measurement position follows a fixed anatomical position within the examination subject moving continuously with the support device. The speed with which the support device is hereby moved is selected such that a travel path during the time of the acquisition of a slice stack is equal to twice the extent of a slice stack. This results in corresponding slices in different slices stacks (for example the respective first, second, . . . slice) being measured identically. Conversely, different slices in a common slice stack are measured differently. In particular, corresponding k-space lines of different slices of a slice stack are hereby measured at different positions within the measurement volume of the magnetic resonance apparatus. Due to the (normally not ideally homogeneous) measurement conditions within the measurement volume, for example inhomogeneities of the basic magnetic field and/or nonlinearities of gradient fields, such measurements at different positions lead to different distortions of the MR images created from the measurement data. Discontinuities thereby arise in complete MR images composed of the individual MR images of the different slice stacks, in particular at the slice stack boundaries, since anatomically adjacent slices that were associated with different slice stacks occupy different positions within their respective slice stack.
This problem does not occur in special 2D axial measurements with continuous movement of the examination subject, for example what is generally known as the “Sliding Multislice” (SMS) technique that, for example, is described in the article by Fautz and Kannengieβer, “Sliding Multislice (SMS): A New Technique for Minimum FOV Usage in Axial Continuously Moving-Table Acquisitions”, Magnetic Resonance in Medicine 55:363-370 (2006). This will be described in detail in the following.
In the SMS technique, the spatial frequency space (known as k-space) belonging to each slice of the real measurement volume is subdivided into S segments. The number N of slices that are measured during a TR interval of the underlying sequence is set equal to a whole-number multiple p of the number of segments:N=p·S, p≧1  (1)
The slices of the examination region are now divided up into the p groups according to a specific pattern. If p is equal to two, for example, those slices of the examination region with even slice index are associated with the first group, and those slices of the slice stack with odd slice index are associated with the second group. A division of the slices into three or generally p groups ensues in an analogous manner, meaning that each third or, respectively, p-th slice of each slice stack is respectively associated with a group.
Furthermore, what is known as an active volume in the measurement volume of the magnetic resonance apparatus is selected. The extent of the active volume along the movement direction of the continuous movement is designed in the following as the active FOV. In the SMS technique the active FOV has an extent of N slice intervals d. This active FOV is now again subdivided into N/p=S equally large sections along the travel direction of the support device (for example along the z-axis). The number of sections is therefore equal to the number of segments. The extent of a section is precisely p slice intervals d where the slice interval d is the distance between adjacent slices. Each segment is now associated with a section of the active FOV. In this association, segments that contain k-space lines near the k-space center are advantageously associated with sections of the active FOV that have a small distance (in terms of absolute value) in the direction of the travel direction of the support device from the isocenter of the magnetic resonance apparatus.
TS=r·TR is now the time that is required for acquisition of the measurement data of a segment. TR is thereby the repetition time of the sequence used for the acquisition, and r is a whole number that depends on the sequence type. In echo train sequences such as turbo spin echo (TSE) sequences or echoplanar imaging (EPI) sequences, a complete segment is normally read out after a single excitation pulse and r is thus equal to one. In gradient echo sequences such as FLASH (Fast Low Angle Shot) or TrueFISP (True Fast Imaging with Steady state Precession), only one line per excitation pulse is read out and r is thus equal to the number of k-space lines per segment. Furthermore, TR is long enough that N slices can be excited, coded and read out in this time period.
A critical requirement to be able to implement the SMS technique is that the table feed during the acquisition time TS of a segment is precisely p slice intervals d between adjacent slices of the examination region. The table speed is thus:
                              v          table                =                                            p              ·              d                                      r              ·              TR                                =                                    N              ·              d                                      S              ·              r              ·              TR                                                          (        2        )            
If this requirement is satisfied, the SMS measurement is implemented as described in the following for illustration with regard to FIG. 4:
The progressing positions in the z-direction (z-axis, here the direction of the continuous travel of the examination region; position z=0 corresponds to the center of the magnetic resonance apparatus) of three slice stacks St1, St2, St3, each made of N=8 slices, are schematically plotted against multiples of the acquisition time of a segment TS in a diagram in FIG. 4. The division of the slices of the examination region into slice stacks here serves merely as an illustration. The first slice stack St1 of the examination region enters directly into the active FOV of the magnetic resonance apparatus at the beginning of the measurement. Due to the conditions of the formulas (1) and (2), precisely p slices 1, 2 (here p=2) of the first slice stack St1 enter into the first section S1 of the active FOV of the magnetic resonance apparatus during a first time interval t1 of duration TS. During the first time interval t1=TS, the k-space segment that is associated with the first section 51 of the active FOV is measured in these p slices 1, 2 of the first slice stack St1. In the second time interval t2=2TS, these p slices 1, 2 of the first slice stack St1 enter into the second section S2 of the active FOV and the k-space segment that is associated with the second section S2 of the active FOV is acquired for the p slices 1, 2 of the first slice stack St1. During the same second time interval t2, the next p slices 3, 4 of the first slice stack St1 (generally the p slices with slice index p+1, . . . , 2p) enter into the first section 51 of the active FOV. For these p slices 3, 4, the k-space segment that is associated with the first section 51 is acquired during the second time interval t2 etc.
During the S-th interval of duration TS (here S=4), the last p slices 7, 8 of the first slice stack St1 (generally the p slices with slice index N−p, . . . , N) enter into the first section S1 of the active FOV, and the first p slices 1, 2 of the first slice stack St1 are located in the last section S4 of the active FOV. The data of the first p slices 1, 2 of the first slice stack St1 are subsequently completely acquired. During the next time interval—thus after S+1 time intervals of duration TS, here t5=5TS—the first p slices 1, 2 of the first slice stack St1 have left the active FOV and the first p slices 1, 2 of the second slice stack St2 enter into the first section S1 of the active FOV etc. It is noted that from the S-th time interval onwards measurement data of N segments in total are acquired per time interval TS or, respectively, that N slices are excited per repetition time TR.
Furthermore, it is noted that the most important sequence techniques that are compatible with the SMS technique are T1-weighted gradient echo sequences and T2-weighted turbo spin echo sequences. In both sequence techniques the acquisition of the measurement data for an MR image follows after multiple excitation pulses (“multi-shot techniques”), and the acquisition duration per MR image is long relative to typical time constants of human breathing (these are in the range from 3-10 seconds, for example). Therefore an acquisition of measurement data in the region of the abdomen and the lungs (thus in regions of a patient that are affected by breathing motion) cannot ensue with an SMS technique without additional measures.
In general, to avoid movement artifacts in examination regions affected by breathing that are caused by the breathing of the patient to be examined it is often necessary that the acquisition of measurement data in such an examination region affected by breathing motions of the patient is conducted under what is known as respiratory triggering. Regions affected by the breathing motion of the patient (for example regions near the lungs or the diaphragm) are at least part of the examination region to be examined, for example in whole-body examinations (for example in what are known as “screenings” in which, for example, persons without disease symptoms are examined from head to toe for possible undetected illnesses or their precursor stages) or in other examinations of, for example, the torso or portions of the torso of a patient to be examined.
In the case of respiratory triggering, the acquisition of the measurement data is synchronized with the quasi-periodical breathing movement so that the acquisition respectively occurs in an identical breathing phase. The periodic measurement pauses that thus occur, the durations of which depend on the individual breathing of the patient, are however not compatible with acquisitions given continuous travel of the support device (and therefore of the examination region). Therefore, this type of acquisition is only possible with a stationary support device. If an examination region to be examined is larger than a measurement volume of the magnetic resonance apparatus that is used, the examination region must be organized into sub-examination regions that fit into the measurement volume and are successively driven into the measurement volume (for example by means of the support device) in order to acquire respective measurement data there given a stationary support device.
Such a step-by-step acquisition of measurement data from sub-examination regions of an examination region given a respectively stationary support device is also possible if the patient holds his or her breath, instead of by respiratory triggering. Each sub-region is hereby traversed in the measurement volume, and at the start of the acquisition the patient is asked to hold his breath until the acquisition of the measurement data for this sub-examination region has concluded. After acquisition of the measurement data of the sub-examination region the patient can breathe until the next sub-examination region is moved into the measurement volume and the acquisition of measurement data of this sub-examination region begins. However, it is hereby problematical that the position of the sub-examination region can be shifted depending on how strongly the patient has inhaled or, respectively, exhaled before each holding of his breath, which can lead to gaps or overlaps between the examined sub-examination regions in the complete MR image of the examination region that is created from the measurement data. For example, if a lesion is located in such a gap, this can be overlooked in the examination.
In order to enable such an interruption of the measurement and the travel and a later continuation of travel and measurement in acquisition techniques with continuous travel, sub-regions of the examination region that should be measured while the breath is held would have to be selected before the start of the measurement. In the aforementioned measurement techniques with continuous travel and organization of the examination region into slice stacks that are measured sequentially, such a sub-region may only comprise whole slice stacks. That only complete slice stacks can be selected in a sub-region given whose measurement the breath should be held severely limits the freedom in this selection. Moreover, the measurement is less efficient since a slice stack can contain slices that are affected by the breathing motion and others that are not affected or are only slightly affected by the breathing motion. After the sub-regions of the examination region are established, the magnetic resonance apparatus would have to be controlled such that the continuous travel and the measurement upon reaching such a sub-region are interrupted and are only continued again after administering a breath-hold command.
In the SMS technique described above, an additional problem that can drastically reduce the efficiency of the measurement occurs given the division of the slices into sub-regions that would have to be measured completely during a breath-hold interval. Given an interruption of the continuous travel after all segments of the last slice of the sub-region are measured, slices exist that border the sub-region and that were already partially measured but that have not traversed all sections of the active FOV during the continuous travel and therefore have no longer been completely acquired during the continuous travel. The same applies for measurement data of slices that border the next slice stack after a resumption of the continuous travel. Here slices are located in the active FOV that are associated with the preceding sub-region.
An attempt to nevertheless acquire measurement data of an examination region between diaphragm and pelvis by means of the SMS technique is published in the article by Sommer et al.: “Sliding Multislice MRI for Abdominal Staging of Patients With Pelvic Malignancies: A Pilot Study”, Journal of Magnetic Resonance Imaging 27:666-672 (2008). There the acquisition of the measurement data is started beginning at the diaphragm, wherein an examined patient should hold his breath for 20 seconds so that the region of the examination region between diaphragm and pelvis that is affected by the breathing motion can be measured during this breath-hold period. For this the speed of the feed of the support device (on which the patient rests during the examination) must be selected high enough in order to have left the region of the examination region that is affected by the breathing movement promptly before resumption of the breathing of the patient. By the temporal placement of the breath-hold interval at the beginning of the measurement it is achieved that a typically deep breathing of the patient before the long breath-hold interval and a breath-hold command to the patient from an operator attending the measurement ensues to temporally match the breath-hold interval with the measurement before the measurement, and thus the entire acquisition of the measurement data can be implemented without interruption under continuous travel of the support device. However, it is furthermore problematical that—as shown above in formulas (1) and (2) for the SMS technique—the speed of the travel of the support device cannot be freely adjusted. Corresponding limitations of the speed of the travel apply in other techniques with continuous travel of the support device. This is primarily relevant due in part to significant limitations of the breath-hold capability in patients. In particular, often ill and/or old patients can hold their breath only for a few seconds. A breath-hold duration of 20 seconds can frequently not be achieved. Given acquisition under continuous travel of the support device, if movement artifacts should be avoided the maximum breath-hold duration must be sufficient to acquire measurement data of the examination region of the patient that is affected by the breathing. The (often short) maximum breath-hold duration thus leads to a higher necessary speed of the travel of the support device in order to be able to leave the examination region affected by the breathing in the short time of the breath-hold duration. This is normally done by a decrease of the resolution in the MR images generated from the acquired measurement data. Irregularities (for example lesions whose size falls below the achievable resolution) can thus not be detected. Additionally, a patient will normally breathe particularly deeply before and/or in particular after holding his breath, whereby movement artifacts at these points in time are disadvantageously intensified.